T.W.
We have wall unit air conditioners and have had to replace them. What you need to do is measure the opening by either removing the original ac or measuring the size of the ac as well as the opening and purchasing another one of the same size. It really isn't a big deal or job, the hard part is when you have one that is at the ceiling like the one we have in our dining room, in which case you will need a couple of strong guys. My husband stuffs foam around the outside and seals the inside with caulking just to keep down on drafts. If you have any questions let me know.